This invention relates to a liquid metal cooled nuclear steam supply system.
In more exact terms, the present invention relates to a nuclear reactor of an intermediate type between the conventional cooling-loop system and the so-called pool type system capable of being connected directly to a steam generator.
It is known that nuclear reactors which are cooled by liquid metal and in particular by liquid sodium or by a sodium-potassium mixture belong to two main types of reactor. On the one hand, there are the loop reactors such as Enrico Fermi in the United States and BN 350 in the Soviet Union. On the other hand, there are the so-called pool type reactors such as EBR2 in the United States, PFR in Great Britain and Phenix in France. All of them have an intermediate non-radioactive sodium system transferring heat from the main reactor vessel to the steam generator. In the second type of reactor, the primary liquid metal cooling circuit is entirely located within the vessel of the nuclear boiler. In more precise terms, this means that the reactor core neutron shielding, the primary pumps and the primary heat exchangers are all located within the reactor vessel itself. In the present state of the technique, this design concept makes it necessary to provide an intermediate cooling circuit for each heat exchange loop, the primary heat exchanger or so-called intermediate exchanger being associated by means of a secondary sodium system with a secondary exchanger or steam generator which carries out heat transfer between the secondary sodium and the water or steam which serves to actuate the turbines. Furthermore, the reactor comprises an outer vessel or so-called "main vessel" and an inner vessel or so-called "primary vessel". Said inner vessel which provides effective containment for the reactor core essentially serves to separate the "hot" liquid metal which leaves the reactor core from the "cold" liquid metal which leaves the heat exchangers and is reintroduced at the core inlet by the primary pumps. The French Phenix reactor can be mentioned as an example of this design concept.
In the first type of liquid metal cooled reactors or in other words the loop reactors, the reactor core neutron shielding, the intermediate heat exchangers and the primary pumps are located outside the inner reactor vessel which contains the reactor core.
An alternative design of loop reactor was disclosed in British Pat. Nos. 985,463 and 985,464. In this case the main vessel contains the reactor core alone. The neutron shields (graphite blocks) are located outside said vessel whilst the intermediate heat exchangers and the primary pump (and not the unit comprising heat exchanger, pump and steam generator) are housed within an auxiliary vessel which is connected to the main vessel by means of a duct provided with a coaxial pipe. Small-diameter pipes serve to connect the pump discharge outlet to the reactor diagrid and pass through the annular space of the coaxial pipes. No tank for storage of sodium and argon is provided and it is apparent that, unless the dimensions of the auxiliary tanks are considerably increased, the reserve supply of cold coolant is distinctly smaller than in the case of a semi-integrated reactor whose thermal inertia is much higher. The method employed for supporting the main vessel and the auxiliary vessels is also different and more conventional since they are suspended from the general biological shield roof of the reactor.
The invention is more especially concerned with the first type of liquid metal cooled reactor which has been mentioned but in which the secondary liquid metal circuit is dispensed with, that is to say reactors in which heat exchange takes place directly between the liquid metal which has passed through the reactor core and the water in the form of liquid and then steam.
The general arrangement of a cooling circuit of this type has already been described in French patent Application No. 77 03192 filed on Feb. 4th, 1977 in the name of the present Applicant in respect of "A pump and heat exchanger unit". This Application describes in particular a heat exchanger and pump unit for direct heat exchange between the primary liquid metal and the water.